This invention relates to an apparatus for measuring a signal level and, more particularly, to an improvement for automatically calibrating with high precision, the gain of a measuring apparatus having frequency selectivity.
Normally, in a conventional signal level measuring apparatus such as an electric field intensity measuring apparatus having frequency selectivity, when a reception frequency changes, the gain of the RF amplifier and the conversion ratio of the frequency converter, which constitute the level measuring apparatus, vary even if the input signals have an identical level. As a result, level measurement with high precision cannot be performed.
In order to prevent the above problem, a reference signal from a reference signal source is supplied to the level measurement apparatus to perform gain calibration before measurement.
FIG. 1 shows an example of a conventional level measurement apparatus having such frequency selectivity. Reference numeral 1 denotes a measuring signal input terminal; 2, a reference signal generator for generating a calibration signal; 3, a high-frequency (RF) amplifier; 4, a frequency converter; 5, a local oscillator; 6, an intermediate frequency (IF) amplifier; 7, a filter for limiting a passband of an input signal; 8, a detector; 9, a gain controllable low-frequency amplifier; and 10, an indicator.
It should be noted that reference symbol SW denotes a switch for switching between measurement and calibration.
In the level measurement apparatus having the arrangement described above, the switch SW is set in the calibration (CAL.) side before actual measurement is performed while the apparatus is tuned to a signal frequency to be measured. A signal having the same frequency as that of the signal to be measured is generated from the generator 2 to control the gain of the apparatus. The switch SW is set in the measuring signal input terminal 1 (MEAS.) side, and the level of the signal being subjected to measurement is read at the indicator 10.
In this case, the generator 2 comprises a reference sinusoidal wave generator for generating a sinusoidal wave having the same frequency as that of the signal to be measured. However, the measuring apparatus requires complicated manual calibration and the reference signal generator has a number of expensive components. When such a generator is incorporated in the measuring apparatus, the apparatus becomes large and heavy, resulting in inconvenience.
In order to resolve the above problem, a pulse generator is used as the generator 2 to generate a plurality of harmonic waves having a uniform level.
In this method, as shown in FIG. 2A, the generator 2 is arranged so that many harmonic waves (signals) f1, f2, f3, . . . having a given level and a spectral interval .DELTA.f are simultaneously inserted in a frequency selection passband BW1 of a level measurement apparatus. The repeating frequency of the pulse generator need not be strictly set. The pulse generator can be easily incorporated in the level measurement apparatus.
However, according to this method, complicated manual calibration is required, and a plurality of harmonic signals are inserted during calibration, thereby presenting the following drawbacks which adversely affect measurement precision.
(1) When a passband BW2 of the signal to be measured is changed by the filter 7, as shown in FIG. 2B, the number of harmonic waves of the pulses falling within the passbands I and II of the filter 7 is changed, because, relation BW2&gt;.DELTA.f is established since the fundamental frequency of the harmonic wave is very low. As a result, the level of the calibration signal source is changed.
(2) Since a signal of considerable magnitude having a wide harmonic wave distribution range is applied, circuits, excluding the passband limiting filter circuit, are saturated, and the input/output characteristics often cannot be linear (FIG. 2A).